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Unformatted text preview: Analysis of commercial vitamin C tablets Objective To determine the vitamin C content in commercial vitamin C tablets by titration between vitamin C (ascorbic acid) & iodine solution. Hence, compare this data with the manufacturer's specification. Principle In this analysis, certain amount of iodine solution, which is known in excess, is added to acidified ascorbic acid solution. Brown iodine can be easily reduced by acidified ascorbic acid to form colourless iodide ion: Equation I ← The excess iodine is then back titrated by standard sodium thiosulphate solution, by using a burette and starch solution (used as end point indicator) Similarly, iodine can also be decolourized by thiosulphate ion: 2S 2 O 3 2- + I 2 → S 4 O 6 2- + 2I- ← Equation II Any iodine present will react with starch to form a blue-black complex. However, when all available iodine has been reacted, the blue-black complex becomes colourless which signals the end-point. Hence, the mass of ascorbic acid reacted by iodine can be calculated. Chemicals vitamin C tablet(s), NaIO 3 , 1M NaI solution, 0.5M H 2 SO 4 , approximately 0.06M Na 2 S 2 O 3 , freshly prepared starch solution Apparatus electronic balance, beaker, volumetric apparatus, pipette, glass rod, white tile, dropper, measuring cylinder Procedure 1.> Accurately weigh 0.6 to 0.7g sodium iodate, NaIO 3 and record its mass. 2.> NaIO 3 is then dissolved in deionized water and is made up to 250cm 3 in a volumetric flask. 3.> Sodium thiosulphate, Na 2 S 2 O 3 , is standardized by part of standard NaIO 3 solution is shown below: (a) Pipette 25cm 3 of NaIO 3 solution into a conical flask and about 5cm 3 of 1M sodium iodide, NaI, together with 5-10 cm 3 of 0.5M sulphuric acid, H 2 SO 4 , are also added. Then, the mixture is immediately titrated with approximately 0.06M Na 2 S 2 O 3 solution. Repeat this step for 3-4 times to obtain a more accurate titration data....
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- Spring '11